One type of electric motor has a rotor provided with permanent magnets and a stator displaying saliency. Such an electric motor produces a cogging torque. The cogging torque represents a torque fluctuation occurring when the motor is rotated without providing a drive current. The cogging torque results from an attractive force between the pole of the rotor and the teeth (poles) of the stator. When a drive current of the motor is provided, the drive current also causes torque ripple. The torque fluctuation i.e. the cogging torque or the torque ripple associated with rotation of the rotor is particularly conspicuous in a concentrated-winding IPM motor (an interior permanent magnet motor). For example, in a type of motor mounted in a vehicle, torque fluctuation can be a source of undesirable vibration for a driver.
Tokkai 2001-69697 published by the Japan Patent Office in 2001 discloses a method termed “skew” of suppressing torque fluctuation. When a cylindrical stator is formed using laminated steel plate, skew represents a method of constructing an inclined salient pole (not parallel to the axial direction) by laminating steel plates with an extremely small angular deviation with respect to a circumferential direction (or a peripheral direction). Skew takes into account the production of torque fluctuations mainly dependent on the relative position of the stator and the rotor. Since a torque waveform from one steel plate displays a phase deviation from that from another steel plate, overall torque fluctuation is reduced by superimposing torque waveforms from all laminated steel plates.